An Australian employee of the G4S security firm on Manus Island says PNG police stood back and allowed locals to break into the compound last month, did nothing to stop them beating detainees, and in some cases participated in the violence and intimidation.

Some expat G4S staff have since left the island suffering from serious post-traumatic stress.

A spokesperson for Immigration Minister Scott Morrison says there are three investigations underway into the Manus Island incident, including a full independent inquiry headed by former senior public servant Robert Cornall.

"These investigations seek to establish the events on Manus Island that evening and we will await the outcome of these reviews," the spokesman said.

"The Government has taken steps with the PNG government to ensure there is information sharing, cooperation and convergence as appropriate to get the clearest possible picture of what occurred that night."

Meanwhile, one staff member told the ABC the situation is so tense that asylum seekers are refusing to have anything to do with local PNG staff.

The Australian G4S employee witnessed "a frenzy of out-of-control violence", telling the ABC that after a second night of protests at the detention centre, G4S staff under attack from rock-throwing asylum seekers asked to withdraw from the facility and agreed to hand over security to PNG police.

The source says PNG police then fired five or six warning shots and stood back as enraged locals poured into the compound over a back fence.

"Our guys were sheltered behind a container and when it came to a point when they couldn't do any more, someone said we need to withdraw," the guard said.

The negotiations that came over the radio raised concerns about handing over control to the PNG police.

"'Do you just want to hand over to the PNG police?' they were asked," the guard said.

"The answer came back over the radio: 'Yes, hand it over to them.'

"And the person who had asked said: 'Are you sure? Do you know the consequences of that?'

"And they said: 'Yes, hand it over'."

Local G4S staff were the first in, followed by local contract staff.

"We saw them going in with machetes. They had anything they could pick up - rocks, sticks, the poles from the exercise weights," the guard said.

"Once they knocked people to the ground, they were stomping on their heads with their boots.

"A day later you could still see guards and staff and cleaners walking around with blood on their boots."

One of the injured asylum seekers has also detailed what happened to him in an interview recorded from hospital a few days after the attack.

He told the ABC:

"Six people were on top of me ... hitting me. At the end I was holding my hand to my face to protect my face form being hit. Then **** came ... I know him personally. He kicked me with his boots ... on my face."

Contrary to the PNG police incident report that describes it as a confrontation between G4S guards and transferees that saw guards being seriously assaulted, the G4S employee says that once the violence began, PNG police did nothing to try and control the situation and some even joined in.

"The police went from room to room as well and held guns to people's heads and said, 'If you don't give me your cigarettes, we're going to shoot you'," the guard said.

Some locals and expats were also inside trying to get transferees out and protect others with shields.

One expat guard lay across a transferee to prevent him being shot.

'There was blood everywhere'

The eyewitness the ABC has spoken to says locals and police went systematically through the compound from room to room.

"There were guys who wet themselves and shit their pants out of total fear," the witness said.

"There are handles broken where they've tried to get in and the transferees have been holding the doors closed and they've snapped the handles off and they've tried to get in with machetes.

The asylum seekers are asking them who they can trust. Who's going to keep them safe?

G4S employee

"The wharf was full from one end to the other and there were puddles of blood everywhere," the witness said.

"They did an emergency tracheotomy out in the open ... there were people moaning ... it was terrible. Everyone pitched in to do what they could, you just needed every single person who was there."

The G4S guard says the transferees now feel so unsafe that when they tried to bring in local cleaners the day after the incidents, it just about started another riot. The asylum seekers want nothing to do with any of the locals.

The expat staff say the atmosphere is so tense that it will not take anything much to kick off another round of unrest.

"The asylum seekers are asking them who they can trust. Who's going to keep them safe? You just don't know if it's going to happen again," the witness said.